Porter, J.H. 1990. Experimental evaluation of nitrogen
saturation in a northern hardwood forest. M.S. Thesis, State
University of New York, College of Environmental Science and
Forestry, Syracuse, 106 pp.

Abstract: The goal of this research was to
induce nitrogen saturation in a forest stand, and to evaluate
saturation-related changes in biogeochemistry. Of particular
concern was the possibility that leaching of nutrient cations
would be accelerated under conditions of elevated nitrate loss.
Following two years of nitrogen loading at rates up to twelve
times ambient deposition, there was a small increase in nitrogen
leaching below the root zone, but no increase in nutrient cation
leaching. Treatment plots remained a net sink for both ammonium
and nitrate. These results suggest that atmospheric deposition of
nitrogen will not accelerate the loss of nutrient cations from
this northern hardwood forest. However, nitrogen saturation was
not achieved (plots remained net sinks for N), hence any
conclusions regarding cation loss may be premature. Continued
addition of N might eventually cause N saturation, with
associated changes in cation leaching rates.

The fact that these plots remained net sinks for nitrogen
despite increased in put of N suggests the conclusion that this
site was "nitrogen-deficient." This does not
necessarily mean forest productivity would increase upon addition
of nitrogen. Changes in the C:N ratio of microbes and soil
organic matter potentially could account for much of the
immobilization of nitrogen input to the system. It might be
better to consider this system as having a high "nitrogen
retention capacity," as this avoids the implication that
productivity is limited by nitrogen.

Although net flux of ions did not change, nitrogen addition at
rates 3-12 times the ambient deposition rate did cause some
changes in soil solution chemistry. The nature of the changes
varied with the rate of N addition. At 15 cm depth under low
treatment, Ca2+ concentration was lower than on
reference plots and NO3 concentration was unchanged,
while under high treatment Ca2+ and NO3
concentrations were significantly and substantially higher than
on reference plots. Microbial processes were also affected
differentially: low treatment caused substantial inhibition of
nitrification but had little impact on ammonification, while high
treatment resulted in negative rates for both processes. These
results indicate that biogeochemical changes associated with
nitrogen loading are not simply threshold dependent, with changes
beginning above some critical load and increasing in magnitude
with increasing loading. Rather, the amount of N available
influences both the nature and the magnitude of biogeochemical
processing of N and, consequently, affects the dynamics of other
elements in the soil.

As with all experiments, it is important to realize this
experiment necessarily involved somewhat unrealistic conditions,
hence all conclusions must be considered captiously. The use of a
single site, a single treatment (albeit at two levels), and a
short time period relative to natural successional time frames
preclude making definitive statements regarding the effects of
atmospheric deposition of nitrogen on northern hardwood
ecosystems.